1. Field of the Invention
The invention relates to rotary drum mixer constructions for use in the making of bituminous paving mixtures and to the method of making such paving mixtures. More particularly, the invention relates to a method of making an asphalt paving mixture in a rotary drum using a low-velocity heated gas stream which moves through the drum with a velocity insufficient to entrain aggregate fines, resulting in reduction of a discharge of the fines into the atmosphere, eliminating the need of auxiliary emission control equipment heretofore required to meet clean air standards.
2. Description of the Prior Art
Bituminous paving mixtures for asphaltic concrete surfaces have heretofore been produced by drying a mixture of aggregate in an inclined rotary dryer unit, feeding the dried aggregate into a screening unit capable of separating the dried aggregate into various sizes, and then storing the dried and separated aggregates into hot bins. The sized aggregates then are withdrawn from the hot bins in the proper proportions by weight or volume and introduced into a pug or mixing mill where they are mixed with a predetermined amount of bitumen to produce the bituminous or asphaltic paving mixture. Such systems are referred to in the trade as "batch" type plants.
Burner systems utilized by such "batch" plants have been of the type whereby only a small amount of the air which is necessary for total fuel combustion is produced by the burner blower (generally 30% of total air). The balance of the air which is necessary for combustion is furnished by secondary air blowers, which are usually in the form of exhaust fans. These exhaust fans subject the drum dryer to a negative pressure which results in large quantities of dust or fines, which are inherent in the aggregate, being discharged into the atmosphere. Nearly all of the minus 200 mesh particles are lost through the exhaust system and discharged into the atmosphere. Such discharge makes it necessary for dust collectors and wet scrubber systems to be added to the asphalt plant in order to meet governmental clean air requirements. Even with the use of these stringent pollution controls, costing many thousands of dollars, it is still very difficult to meet these requirements.
Other asphalt processing systems use an inclined rotary drum as a combination drying and mixing unit. In these systems aggregate is introduced into the upper end of a heated drum and coated with an emulsion or asphaltic material which is sprayed or pumped into the stream of moving aggregate. The asphalt coated aggregate then is discharged as a finished paving mix from the drum ready for use. Since the rescreening, mixing towers and batching systems are eliminated in this rotary drum type plant, it may be produced for sale at a lower cost, resulting in lower investments for asphalt producers.
The burner and exhaust systems of these drum mixers are similar in most respect with the systems in use in standard batch plants. Although the injections of liquid asphalt in the drum traps a large portion of the aggregate fines, the exhaust fan still withdraws a large percentage of these fines from the drum and mix. This requires expensive emission control equipment which adds thousands of dollars to the initial purchase price and maintenance of these units.
Examples of such rotary drum asphalt plants and other types of asphalt and material mixing systems are set forth in U.S. Pat. Nos. 1,240,481, 2,188,798, 2,028,745, 2,626,875, 3,423,222, 3,614,071, and 3,832,201.
Recent asphalt plants and systems have been concerned with the elimination or reduction of aggregate fines and other particulate matters from being discharged into the atmosphere in order to meet governmental clean air standards, and to reduce the amount of pollution control equipment required to meet such standards. Systems such as shown in U.S. Pat. No. 3,614,071 spray liquid asphalt onto the moving aggregate within a mixer drum to coat the aggregate with the asphalt and particularly to trap the aggregate fines in the mix before the fines are emitted into the atmosphere through the exhaust stack. Other systems, such as shown in U.S. Pat. No. 3,832,201 coat the aggregate with liquid asphalt while the aggregate is in a cold, wet condition prior to drying the aggregate in the drum in an attempt to eliminate or reduce emission of the aggregate fines into the atmosphere.
Such systems, however, still require the use of exhaust blowers to draw air into the inlet end of the rotary drum, through the drum, and then out of an exhaust stack. The heated gas stream which moves through the drum, also has a relatively high velocity due to the use of these exhaust blowers. These exhaust blowers increase considerably the initial plant equipment cost as well as the operating cost, due to the amount of electricity required to operate the blower. These exhaust blowers usually require a large horsepower motor which consume a large portion of the electrical energy requirements of an asphalt processing plant.
No asphalt processing system or method of which I am aware controls the emission of aggregate fines into the atmosphere by providing a heated gas stream which moves in a lazy-like fashion through the drum, which gas stream has a velocity sufficiently low to prevent suspension or entrainment of the aggregate fines in the airflow by using a natural draft stack at the discharge end of the drum and by controlling the air-fuel ratio to the drum burner, thereby eliminating exhaust blowers or fans of any type.